1. Field of the Invention
This invention relates to electrical devices comprising PTC conductive polymer compositions.
2. Introduction to the Invention
Conductive polymers are well known. They comprise a particulate conductive filler which is dispersed in, or otherwise held together by, an organic polymer. They can be used in circuits in which current passes through them, eg. in heaters and circuit protection devices, and in such use they may exhibit what is known as PTC (positive temperature coefficient) or ZTC (zero temperature coefficient) behavior. The term "PTC behavior" is usually used in the art, and is so used in this specification, to denote a composition which, in the operating temperature range, has an R.sub.14 value of at least 2.5 or an R.sub.100 value of at least 10, preferably both, and which preferably has an R.sub.30 value of at least 6, where R.sub.14 is the ratio of the resistivities at the end and the beginning of the 14.degree. C. temperature range showing the greatest increase in resistivity, R.sub.100 is the ratio of the resistivities at the end and the beginning of the 100.degree. C. temperature range showing the greatest increase in resistivity, and R.sub.30 is the ratio of the resistivities at the end and the beginning of the 30.degree. C. temperature range showing the greatest increase in resistivity. The term "ZTC behavior" is usually used in the art, and is so used in this specification, to denote a composition which does not show PTC behavior in the operating temperature range; thus the term is used to include (a) compositions which show no substantial change in resistivity over the operating temperature range (eg. from room temperature to 100.degree. C.), (b) compositions which show substantial increases in resistivity over the operating temperature range but still do not have R.sub.14, R.sub.30 or R.sub.100 values as specified above, (c) compositions which show substantial decreases in resistivity over the operating temperature range [often denoted NTC (negative temperature coefficient) compositions], and (d) compositions as defined in (a), (b) and (c) which exhibit PTC behavior at temperatures above the operating temperature range.
Documents describing conductive polymer compositions and devices comprising them include U.S. Pat. Nos. 2,952,761, 2,978,665, 3,243,753, 3,351,882, 3,571,777, 3,757,086, 3,793,716, 3,823,217, 3,858,144, 3,861,029, 3,950,604, 4,017,715, 4,072,848, 4,085,286, 4,117,312, 4,177,376, 4,177,446, 4,188,276, 4,237,441, 4,242,573, 4,246,468, 4,250,400, 4,252,692, 4,255,698, 4,271,350, 4,272,471, 4,304,987, 4,309,596, 4,309,597, 4,314,230, 4,314,231, 4,315,237, 4,317,027, 4,318,881, 4,327,351, 4,330,704, 4,334,351, 4,352,083, 4,361,799, 4,388,607, 4,398,084, 4,413,301, 4,425,397, 4,426,339, 4,426,633, 4,427,877, 4,435,639, 4,429,216, 4,442,139, 4,459,473, 4,481,498, 4,476,450, 4,502,929; 4,514,620, 4,517,449, and 4,534,889; J. Applied Polymer Science 19, 813-815 (1975), Klason and Kubat; Polymer Engineering and Science 18, 649-653 (1978), Narkis et al; and commonly assigned U.S. Ser. Nos. 601,424 now abandoned, published as German OLS No. 1,634,999; 732,792 (Van Konynenburg et al), now abandoned, published as German OLS No. 2,746,602; 798,154 (Horsma et al), now abandoned, published as German OLS No. 2,821,799; 141,984 (Gotcher et al), now abandoned, published as European Application No. 38,718; 141,988 (Fouts et al), now abandoned, published as European Application No. 38,718, 141,989 (Evans), published as European Application No. 38,713, 141,991 (Fouts et al), published as European Application No. 38,714, 150,909 (Sopory), published as UK Application No. 2,076,106A, 250,491 (Jacobs et al) published as European Application No. 63,440, 274,010 (Walty et al), now abandoned, 300,709 and 423,589 (Van Konynenburg et al), published as European Application No. 74,281, 369,309 (Midgley et al), published as European Application No. 92,406, 483,633 (Wasley), 606,033 (Leary et al), published as European Application No. 119,807, 599,047 and 598,048 (Masia et al), published as European Application No. 84,304,502.2, 524,482 (Tomlinson et al), published as European Application No. 84,305,584.7, 534,913 (McKinley), now abandoned, 552,649 (Jensen et al), published as European Application No. 84,307,984.9, 573,099 (Batliwalla et al) and 904,736 (Penneck et al), published as UK Patent Nos. 1,470,502 and 1,470,503, 650,918 (Batliwalla et al, 650,920 (Batliwalla et al,), 663,014 (Batliwalla et al,), 735,408 (Batliwalla et al,), 650,919 (Batliwalla et al,), 650,921 (Kheder,), 711,790 (Carlomagno,), 667,799 (Frank,), 711,908 (Ratell,), 687,120, (Leary et al,), 691,291 (Hauptly, et al,), 711,907 (Ratell,), 711,909 (Deep et al,), 720,118 (Soni et al,), 711,710 (Bliven,), 711,910 (Au et al,), 716,780 (Sandberg,), 735,409 (Batliwalla et al,), 741,657 (Morrow et al,), 744,170 (Lahlough et al,), and 764,894 (Batliwalla et al,). The disclosure of each of the patents, publications and applications referred to above is incorporated herein by reference.
Devices of particular interest are heat-recoverable articles which comprise conductive polymer elements and which can be recovered by passing current through the conductive polymer, thus generating the heat needed to cause recovery; preferably the conductive polymer comprises a PTC conductive polymer element which controls the heating process so that overheating cannot take place during or after recovery. Such articles can be used to provide an environmental barrier around a splice in a telephone cable or other substrate. Reference may be made for example to U.S. Pat. Nos. 4,223,209, 4,330,703, 4,421,582, and British Pat. No. 1,265,194, the disclosures of which are incorporated herein by reference. As is well known in the art, a polymeric article can be rendered heat-recoverable by cross-linking it, deforming the cross-linked article at a temperature above its melting point, and cooling the article in the deformed state (see for example U.S. Pat. Nos. 3,086,242 2,027,962, 3,086,242, 3,253,618, 3,253,619, 3,379,218, 3,455,336 and 3,770,556, the disclosures of which are incorporated herein by reference). A limiting factor in the development of electrically powered recoverable articles is the fact that when a cross-linked PTC conductive polymer is stretched, the increase in resistivity around the switching temperature (T.sub.s) decreases; the greater the stretching, the less the resistivity increases. For example, in many cases, if a cross-linked PTC conductive polymer is stretched more than 2X (ie. to twice its original dimension), as would be desirable, for example, in making heat-recoverable articles as disclosed in U.S. Pat. No. 4,421,582, the conductive polymer ceases to exhibit PTC behavior or at any rate ceases to show an increase in resistivity which provides adequate assurance against runaway heating U.S. Pat. Nos. 4,177,446 and 4,223,209 seek to overcome this drawback by means of an apertured PTC conductive polymer heater which can undergo dimensional change by changes in the shape of the apertures and which is attached to a non-conductive heat-recoverable article. This is a useful approach, but it suffers from the disadvantage that the heater itself does not provide a barrier against moisture etc. after recovery.